The term Joomla Developer is a little bit ambiguous and broadly describes two groups of people. The first group are what I like to call Site Builders, they use Joomla to create websites, often leveraging third party templates and extensions, and making minor customisations to suit the requirements of the project. The other group of people are the coders/programmers that actually write PHP code, and are the ones who create the third party templates and extensions, as well as contribute code to the Joomla project. So which type of developer are you, and what are you looking for?
If you love building websites with Joomla, then the Joomla Extension Directory is a good place to start, so you can find all the third party extensions that you need for your website.
Coder / Programmer
If you love coding, then a good place to start is with the Joomla! Developer Network, where you can find out the current development status of the Joomla project, report bugs on the Issue Tracker and access other useful information such as the project roadmap.
No matter how you use Joomla, I highly recommend getting involved in the community, as you will make new friends, learn from other Joomla enthusiasts, and contribute back to make Joomla even more successful and widely used in Australia. The best way to start is to get in touch with your local JUG.